We have a 3 PH 480 Arc welder that is being used to weld some pieces on a conveyor chain. The employee welding pieces on is using an overhead crane/trolley type chain hoist. It is also 480V 3 PH and is supplied from same source as welding machine.

The work (conv chain) is attached to work (ground) lead of the welding machine. The stinger does not have to be near piece for this to happen and isn't involved. If work is touched by the hook of chain hoist he is using, it sparks violently. The machine is set to dc+ on electrode, so that would mean work piece is dc-, correct? DC flows from - to +, so I reversed polarity of machine, but it still sparked when work was touched by chain hook of chain hoist.

I did not read any voltage from chain hoist hook to ground. I tried switching power off at breaker on hoist, same result. Ground (work) lead of machine must be making good contact, but what causes this to happen? I am stumped.

 

I would say that both are simply grounded. Since they are at the same potential, hooking the ground clamp to one makes the stinger hot to both.

Regards
cww

 

One should always use a sling (not steel) chocker when using overhead crane for support in welding and vise-versa. There are too many ways for grounding to transfer through the crane causing compounding issues.

 

Stinger is not involved, only work (ground) lead from welder and the chain hoist which is referenced to ground of bldg. I get 35 VDC- between bldg and work terminal of welder, 35 with other terminal and 68 VDC between both,.

I get in k-ohms between bldg gnd and either terminal, so terminals are not referenced to ground. Case is though. Is this normal or do I have a potential problem with welding machine?

 

Seems like the center tap of the welder transformer may be grounded. With a full wave bridge rectifier this will divide the voltage as you describe. I have seen this happen when someone replaced a two diode setup with a bridge rectifier, but normally the secondary would be floating. It would be good to get to the secondary of the transformer and look for grounds. The diodes will often mess with your ohmmeter reading. This is for a single phase setup.

With three phase,it would take something more interesting to equally divide the DC output, but I think it's safe to say that, yes, you should have the welder looked at or dig in yourself as there seems to be an unintentional ground on the secondary side. Substitute stator for a rotary machine.

Regards
cww

 

I am concerned about your welding pieces on a conveyor chain. The ground path on a conveyor chain is through the chain pivots and through the sprockets, then to the shafts, and then to the bearings. You can end up welding the bearings to the shafts.

I am suspicious of the grounding of the overhead chain hoist and would again verify that the hoist is grounded. A nylon sling is highly recommended to isolate the chain hoist from being a ground path because again you can weld the bearings.

Again, I would make sure that the welder itself is grounded. Sloppy pigtails are used a lot to hook up welders.

Good Luck